The term "piece goods" is a term of art which refers to a large quantity of identical or at least similar objects which are conveyed and processed in succession while being held individually or in groups. For conveying piece goods as defined above, it is usual to use conveying systems which are closed systems, i.e., conveying systems in which holding means, each holding and conveying one object or one group of objects are moved in a predetermined direction along a conveying path which is closed in itself. On this conveying path, at least one supply station and at least one removal station are provided, and in those stations objects are supplied to holding means or are removed from holding means, respectively. Between the supply station and the removal station, the conveyed holding means are loaded and their load may also be processed. Between the removal station and the supply station, the holding means are empty.
The holding means are, e.g., attached to an endless conveying chain at regular distances from each other, whereby the conveying chain runs in a chain channel and is driven by a suitable drive. However, the holding means can also be loosely connected with each other, i.e., without fixed distances between each other, or they can be individually movable along a rail system which is closed in itself without being mutually connected.
In stations through which or past which objects being held by holding means or empty holding means are conveyed, objects or holding means are acted on, e.g., supply or removal actions, handing-over actions, repositioning actions, processing actions or sensor actions for detecting characteristics of objects or of holding means. These actions are at least partly triggered by active control, either for each object or holding means being conveyed to the station, or for only a specific selection of the supplied objects or holding means, respectively. Control of such actions and synchronization of such actions with the conveying are important control functions in a system in which piece goods are conveyed and processed.
Specific problems concerning the control of conveying and processing of piece goods have, according to the state of the art, led to specific solutions. Stations in systems with a conveying chain are, e.g., controlled by means of a system clock cycle and possibly a fine cycle. For specific actions concerning specific objects or holding means, the clock cycles are counted and relevant cycle numbers are calculated for different positions along the conveying path according to their distances from a reference position. The basic element of control in such a system is the system clock cycle being correlated with the conveying speed.
In systems with very long conveying chains, an overall clock cycle control is no longer accurate enough and is therefore often replaced by local clock cycles and/or by sensor detection of holding means.
For triggering specific actions for specific objects or holding means in systems with holding means which are loosely connected to each other, or with holding means which are fully independent of each other, it is also suggested to equip each holding means with an electronic memory means containing a code or even data regarding characteristics of the held object and to provide suitable reading means for reading the code or memory contents in order to identify the holding means or to directly trigger a specific action.
All known attempts at solving the given problem are designed for specific applications and are either connected with too much effort or are too restricted in their effect for more general application.